The invention relates to a plasma-generating spark plug, used especially for the ignition of internal combustion engines by means of electric sparks.
The spark plug for the internal combustion engine of a motor vehicle, having a substantially cylindrical general form, comprises:                an essentially capacitive lower part comprising:                    two coaxial electrodes: one internal electrode with an axis known as the central electrode (3) and one external electrode known as the shell surrounding the central electrode, and            an electrically insulating block known as the insulator inserted between the central electrode and the shell; and                        an essentially inductive upper part comprising:                    a central mandrel surrounded by a coil, a bottom end part of which surrounds a top end part of the central electrode,            an external casing, and            an insulator inserted radially between the casing and the coil.                        
The publications FR 2 859 830, FR 2 859 569 and FR 2 859 831 relate to a multi-spark plug of this type integrating a series resonator. The single winding of turns of the coil allows a high quality coefficient to be achieved, which is the ratio between the energy stored in the structure and the ohmic and dielectric losses. All the energy is thus stored in magnetic form and transferred to the essentially capacitive part. Moreover, the bottom end part of the coil surrounds the top end part of the central electrode. This electromagnetic overlap zone induces eddy currents in the central electrode, which have the effect of creating extra resistance and thus of reducing the overvoltage coefficient of the coil/spark plug assembly.